There has been extensive use of an image forming apparatus using the electrophotographic process wherein an electrostatic latent image is formed on a photoconductor (image carrier) and is developed with toner. The developed toner image is transferred to a recording medium such as paper and is then fixed on the recording medium.
The development methods for developing an electrostatic latent image using a dry developer includes a one-component development method using toner alone and a two-component development method using both toner and carrier.
In the one-component development method, the toner is passed through the regulating section formed by a toner carrying member and the regulating plate pressed against the toner carrying member, thereby the toner is electrically charged and a desired thin toner layer is obtained. This method provides a simplified configuration, a compact and lightweight structure, and reduces cost of the apparatus. However, toner deterioration is accelerated by the strong stress by the regulating section and the electric charge acceptance of toner tends to be reduced. Further, when the surface of the regulating member as a member for providing the toner with electric charges and the surface of the toner carrying member are contaminated with the toner and external additive agent, the capability of providing the toner with electric charges will be reduced. This will cause a further reduction in toner charge, and will raise fogging or similar problems. For these reasons, the service life of the development apparatus is shorter in general.
In the meantime, in the two-component development method, toner is charged by the frictional charging process through mixture with the carrier. Therefore, this method ensures reduced stress. Further as a surface area of the carrier is large, the carrier has a higher resistance to contamination with the toner and external additive agent. These result in a longer service life of the apparatus. However, when an electrostatic latent image is developed on the image carrier, the image carrier surface is rubbed by the magnetic brush formed by the developer and the marks of the magnetic brush remain on the developed image in the two-component development method. Further, the carrier tends to be deposited on the image carrier. This will cause a problem of image defects.
A so-called hybrid development method is disclosed as a development method (refer to, for example, Unexamined Japanese Patent Application Publication No. H 05-150636), which solves the problems of image defects resulting from the remaining marks of the magnetic brush and others, and realizes high image quality comparable to that of the one-component development method as well as the service life as long as the two-component development. In this hybrid development method, the two-component developer is carried on the developer carrying member. From this two-component developer, only the toner is supplied to the toner carrying member for use in development.
In the hybrid development method, a bias voltage is applied, and toner is supplied onto the toner carrying member from the developer carrying member. The toner layer formed on this toner carrying member is used to develop the latent image on the opposed image carrier.
In the one-component development method or hybrid development method, the amount of the deposited toner in a layer formed on the toner carrying member or specific charge (electric charge per unit mass of the toner layer) depends on the environment where the apparatus is used (temperature, humidity, atmospheric pressure, and others) or number of sheets having been printed. This will cause changes in the state of image formation, particularly in the development characteristics. These changes result in changes in image density, and further in deterioration of the image quality. The solution to this problem requires accurate detection of the changing amount of toner held on the toner carrying member for the purpose of maintaining the image quality.
A method commonly used to minimize the fluctuation in image density is disclosed in the Unexamined Japanese Patent Application Publication No. 2003-173052, for example. In this method, a dummy toner image is developed on the image carrier and the toner amount of the developed toner image is detected at prescribed timings when the toner image (image to be transferred onto the sheet is not formed on the image carrier, for example, before and after image forming operation or during sheet-to-sheet interval in image forming operation. The development bias between the image carrier and toner carrying member is controlled to ensure the toner amount for maintaining the required image density. This method, however, fails to detect the toner amount during the image forming operation.
To solve these problems, Unexamined Japanese Patent Application Publication No. 2008-176236 discloses a method of using the detecting means to detect the toner amount in the toner layer formed on the toner carrying member instead of the toner amount held on the image carrier. This method uses an LED or LD as a light emitting means, which applies light to the toner layer. The reflected light is detected by the light-receiving element such as a photoelectric conversion element, and the absolute amount of the toner layer is obtained from the intensity of this reflected light.
Unexamined Japanese Patent Application Publication No. H06-258949 discloses a method of controlling and maintaining the toner amount held on the toner carrying member at a prescribed level. This control is based on the charge amount of the toner supplied from the developer carrying member to the toner carrying member. This charge amount is obtained by analyzing the current value flowing through the closed loop circuit made up of the toner carrying member, the developer carrying member and an electric power supply apparatus for bias connected to each of them.
In the method disclosed in Unexamined Japanese Patent Application Publication No. 2008-176236, the amounts of light reflected from the toner layer and the toner carrying member surface are measured, therefore, the changes in the amount of the toner cannot be detected in the area where there is too much absolute amount of toner on the toner carrying member. This results in poor detection accuracy and poor image quality. Further, in Unexamined Japanese Patent Application Publication No. H06-258949, the specific charge of toner itself depends on the working environment or the number of sheets having been printed. Thus, an appropriate toner amount cannot be obtained when control is provided based on the result of analysis of the detected current value, and satisfactory image quality cannot be obtained. For example, control is provided in such a way that, when there is an increase in the specific charge of the toner, the toner amount held on the toner carrying member will be reduced. This results in reduced image density.